It is known that an optical access network allows a plurality of users to access a number of broadband services, such as for instance Internet access, video-on-demand and telephone services.
Among the known optical access networks, passive optical networks (briefly termed PON) are becoming even more widespread. Typically, a PON comprises an optical line termination (briefly termed OLT) and an optical distribution network (briefly termed ODN). The ODN comprises a plurality of optical links (typically comprising silica-based single-mode standard optical fibers) and optical splitters arranged so as to form a tree structure whose root is the OLT.
The ODN is suitable for allowing the OLT to exchange traffic with users connected at the far end of the optical links at transmission rates which can be higher than 100 Mbit/s. This advantageously allows the users to share the usage (and therefore the costs) of the OLT, thus allowing them to access broadband data services and broadband telephone services at acceptable costs.
A PON may be used for different applications. In most cases, the OLT is installed at a central office, while the optical links of the ODN are at least partially installed in underground pipes.
For instance, when the PON is used for FTTH (Fiber To The Home) applications, the ODN is arranged so that one of its optical splitters is installed e.g. at the basement of a building. Besides, the optical links radiating from the optical splitter are installed within suitable cable trays, which vertically run through the building from the basement up to the various floors. Each optical link is terminated within a respective apartment by means of a plug. A user wishing to access the broadband services supported by the PON may either buy or lease a suitable optical network termination (briefly termed ONT) and connect the ONT directly to the plug located in his apartment.
On the other hand, when the PON is used for FTTB (Fiber To The Building) applications, the far end of each optical link of the ODN reaches the basement of a respective building. Each optical link is then terminated by means of an optical network unit (briefly termed ONU). Each ONU is suitable for exchanging traffic with the users residing in the building, e.g. by means of a high speed transmission system (such as for instance ADSL or VDSL) using the twisted pairs of the fixed telephone network (PSTN, Public Switch Telephone Network).
A PON may also be employed for other applications, such as FTTC (Fiber To The Curb) and FTTCab (Fiber To The Cabinet). A same PON may also support different applications. For instance, a same OLT may be connected to first optical links supporting FTTH applications, second optical links supporting FTTB applications, and third optical links supporting FTTC or FTTCab applications.
The document “Securing Passive Optical Networks Against Signal Injection Attacks” by H. Rohde et al., Optical Network Design and Modelling, pages 96-100, ISBN 978-3-540-72729-3, discloses to avoid that a single user or a failing terminal degrades (or even disables) a passive optical network for other connected users by sending light permanently or casually. To this purpose, this document discloses to provide each optical link of the PON with a respective optical switch. In case a permanent signal is detected from some ONUs, a controller can firstly detect that such a continuous signal is sent, and then identify the port by briefly disconnecting the users, invoking the switches. Once the attacking port is identified, i.e. when during a switch-off the permanent signal disappears, the corresponding port can be switched off and maintenance personnel can react on the malfunctioning of the ONU.